1. Field of the Invention
The present invention relates to technologies by which to transmit data to radio terminals which are targeted communication parties, and wireless technologies utilizing such transmission technologies, and it particularly relates to a transmission method for controlling the directional patterns of antennas and radio apparatus utilizing the transmission method.
2. Description of the Related Art
In wireless communication, it is generally desired that the limited frequency resources be used effectively. Adaptive array antenna technology is one of the technologies that can realize effective utilization of frequency resources. In adaptive array antenna technology, the amplitude and phase of signals transmitted and received by a plurality of antennas, respectively, are so controlled as to form a directional pattern of the antenna for transmission and receiving. In other words, the apparatuses provided with adaptive array antennas change the amplitude and phase of signals received by their respective antennas, add up the thus changed received signals, and receive the signals equivalent to the signals received by the antenna having the directional pattern corresponding to the variation in the said amplitude and phase (hereinafter referred to as “weight”). In addition, signals are transmitted in a directional pattern of the antenna corresponding to the weight.
One example of weight computation in adaptive array antenna technology is a processing based on the MMSE (Minimum Mean Square Error) method. In the MMSE method, the Wiener solution is known to be the condition for an optimum weight value. Also known is a recurrence formula that converges to the optimum value with smaller amounts of calculation than directly solving the above-mentioned condition. For such recurrence formula, adaptive algorithms, such as RLS (Recursive Least Squares) algorithm and LMS (Least Mean Squares) algorithm, are used (See reference (1) in the following Related Art List, for instance).
3. Related Art List
(1) Japanese Patent Application Laid-Open No. 2002-77015.
With adaptive array antennas, radio signals are transmitted and received according to a directional pattern formed based on a weighting control to find an optimum weight value in response to changes in the radio wave environment in the wireless-communication covered area. Accordingly, there would be little change in the directional pattern if there is no significant change in the radio environment. The radio signals transmitted from a wireless base station apparatus equipped with adaptive array antennas can be received not only by targeted wireless terminal apparatuses, which are intended communication parties, but also by other wireless terminal apparatuses, that are not targeted communication parties, which happen to be located within the range of the directional pattern of the adaptive array antenna used. Particularly in the radio environment where there is little change in the directional pattern of the adaptive array antennas, there are possibilities that the radio signals are continuously received by unwanted radio terminals which are unintended communication parties, thus breaching the security of information contained in the radio signals. Moreover, the above security-related problem is not limited to adaptive array antennas but can also occur with antennas with any fixed directional pattern or non-directional antennas.